Hierarchical WS2-WO3 Nanohybrids with Flower-like p-n Heterostructures for Trimethylamine Detection

The detection of trimethylamine (TMA) is critically important due to its toxic and flammable nature, which poses significant risks to human health and the environment. However, achieving high response, rapid kinetics, selectivity, and low operating temperatures in TMA sensing remains challenging. In this study, WS2/WO3 nanohybrids with flower-like hierarchical structures were synthesized via an in situ sulfurization process, utilizing varying amounts of thioacetamide to control the sulfurization state of WO3. These novel hierarchical WS2/WO3 nanohybrids exhibit remarkable selectivity towards TMA, as well as rapid response and recovery characteristics. Specially, the optimal WS2/WO3 sensor, composed of 5% WS2/WO3 nanohybrids, demonstrates exceptional TMA sensing performance, including a high response (19.45 at 10 ppm), good repeatability, reliable long-term stability, and a low theoretical detection limit (15.96 ppb). The superior sensing capabilities of the WS2/WO3 nanohybrids are attributed to the formation of p-n heterojunctions at the interface, the unique hierarchical structures, and the catalytic activity of WS2. Overall, this work provides a straightforward and versatile approach for synthesizing multifunctional nanomaterials by combining metal oxide micro-flowers with transition metal dichalcogenide nanoflakes for applications in monitoring TMA in complex environments.


Introduction
Trimethylamine (TMA) is one of the volatile organic compounds (VOCs) and is a colorless, flammable gas with a strong and fishy odor.It is naturally produced during the decomposition of organic matter and is commonly found in various biological processes [1].TMA can serve as an essential intermediate in industrial chemical synthesis and is used to produce various products, including pharmaceuticals, pesticides, and surfactants [2].Inhalation of TMA vapors can cause health issues such as coughing, upper respiratory system irritation, breathing difficulties, and lung edema [3].The presence and concentration of TMA are often monitored as indicators of spoilage in food products, particularly fish and seafood [4,5].Additionally, TMA has been detected in human exhaled breath at concentrations ranging from 0.1 to 0.2 ppm, suggesting its potential as a biomarker for renal diseases [6].Therefore, monitoring TMA levels can provide valuable insights into the freshness and quality of seafood products, early disease diagnosis, and environmental monitoring.
In recent years, metal oxide semiconductors such as WO 3 [7], SnO 2 [8][9][10], ZnO [11,12], NiO [13,14], and In 2 O 3 [15,16] have been extensively utilized as sensing materials for detecting various gases in the environment due to their relatively low cost, ease of integration, and broad gas detection capabilities [17,18].Among these, WO 3 stands out due to its The synthesis process of WS 2 /WO 3 nanohybrids with flower-like hierarchical structures is schematically illustrated in Figure 1a.Initially, 2.5 g of H 2 WO 4 was dispersed in a solvent mixture composed of 50 mL deionized water and 34 mL (30 wt%) H 2 O 2 .The solution was stirred at 95 • C in a water bath until the H 2 WO 4 completely dissolved.After cooling, deionized water was added to dilute the solution to 200 mL, resulting in an H 2 WO 4 precursor solution with a concentration of 0.05 M. Subsequently, 0.05 g C 2 H 2 O 4 •2H 2 O and 0.05 g urea were sequentially added to the solvent mixture, which consisted of 7.5 mL of the H 2 WO 4 precursor solution, 31.5 mL CH 3 CN, and 2.5 mL HCl (3 M), and stirred for 30 min.The solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave and reacted at 180 • C for 12 h.Upon cooling to room temperature, the precipitate was subjected to six cycles of ethanol washing and subsequently dried overnight at 60 • C, resulting in WO 3 with flower-like hierarchical structures.To obtain WS2/WO3 flower-like hierarchical structures, the prepared WO3 (115 mg) was dispersed in 30 mL of deionized water and stirred for 1 h.The obtained dispersion was then mixed with appropriate amounts of C2H5NS, corresponding to [S]/[W] molar ratios of 0.03, 0.05, 0.07, and 0.09, respectively, and magnetically stirred for an additional hour.The mixed solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 24 h.After the reaction, the precipitates were collected, sequentially washed with deionized water and ethanol, and dried in a vacuum drying oven at 60 °C to yield the WS2/WO3 composites.The resulting WS2/WO3 composites, synthesized with [S]/[W] molar ratios of 0.03, 0.05, 0.07, and 0.09, were designated as 3%-WS2/WO3, 5%-WS2/WO3, 7%-WS2/WO3, and 9%-WS2/WO3, respectively.

Characterization
The crystalline structures of WS2/WO3 were analyzed using X-ray diffraction (XRD, Shimadzu XRD-6100) with Cu Kα radiation.The morphology of the products was examined by scanning electron microscopy (SEM, ZEISS Ultra Plus, ZEISS, Oberkochen, Germany), which was equipped with energy-dispersive X-ray spectroscopy (EDS) to obtain elemental distribution maps.Further detailed characterization of the microstructures was performed using high-resolution transmission electron microscopy (HRTEM) on a transmission electron microscopy system (TEM, JEOL, JEM-2100F, Tokyo, Japan).The charge carrier transport properties of the products were analyzed by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (Shanghai Chenhua Science Technology Corp., Ltd., Shanghai, China) with a standard three-electrode configuration.A glassy carbon electrode was employed as the working electrode, while a graphite rod and a saturated Hg/HgCl2 electrode served as the counter and reference electrodes, respectively.To prepare the working electrode, the glassy carbon electrode was initially polished with an alumina suspension.Subsequently, 2 mg of the WS2-WO3 nanohybrids was dispersed in a solution containing 97 µL of deionized water, 100 µL of ethanol, and 3 µL of Nafion solution.The resulting mixture underwent ultrasonic treatment for 30 min to achieve a homogeneous ink.Finally, 50 µL of the ink was dropped onto the surface of the glassy carbon electrode To obtain WS 2 /WO 3 flower-like hierarchical structures, the prepared WO 3 (115 mg) was dispersed in 30 mL of deionized water and stirred for 1 h.The obtained dispersion was then mixed with appropriate amounts of C 2 H 5 NS, corresponding to [S]/[W] molar ratios of 0.03, 0.05, 0.07, and 0.09, respectively, and magnetically stirred for an additional hour.The mixed solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and maintained at 160 • C for 24 h.After the reaction, the precipitates were collected, sequentially washed with deionized water and ethanol, and dried in a vacuum drying oven at 60 • C to yield the WS 2 /WO 3 composites.The resulting WS 2 /WO 3 composites, synthesized with [S]/[W] molar ratios of 0.03, 0.05, 0.07, and 0.09, were designated as 3%-WS 2 /WO 3 , 5%-WS 2 /WO 3 , 7%-WS 2 /WO 3 , and 9%-WS 2 /WO 3 , respectively.

Characterization
The crystalline structures of WS 2 /WO 3 were analyzed using X-ray diffraction (XRD, Shimadzu XRD-6100) with Cu Kα radiation.The morphology of the products was examined by scanning electron microscopy (SEM, ZEISS Ultra Plus, ZEISS, Oberkochen, Germany), which was equipped with energy-dispersive X-ray spectroscopy (EDS) to obtain elemental distribution maps.Further detailed characterization of the microstructures was performed using high-resolution transmission electron microscopy (HRTEM) on a transmission electron microscopy system (TEM, JEOL, JEM-2100F, Tokyo, Japan).The charge carrier transport properties of the products were analyzed by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (Shanghai Chenhua Science Technology Corp., Ltd., Shanghai, China) with a standard three-electrode configuration.A glassy carbon electrode was employed as the working electrode, while a graphite rod and a saturated Hg/HgCl 2 electrode served as the counter and reference electrodes, respectively.To prepare the working electrode, the glassy carbon electrode was initially polished with an alumina suspension.Subsequently, 2 mg of the WS 2 -WO 3 nanohybrids was dispersed in a solution containing 97 µL of deionized water, 100 µL of ethanol, and 3 µL of Nafion solution.The resulting mixture underwent ultrasonic treatment for 30 min to achieve a homogeneous ink.Finally, 50 µL of the ink was dropped onto the surface of the glassy carbon electrode and allowed to dry at room temperature.The electrochemical tests were performed in a 0.5 M Na 2 SO 4 solution (pH~7) at room temperature, with a frequency range spanning from 0.1 Hz to 10,000 Hz.

Fabrication and Measurement of Sensors
The fabrication process of the gas sensor was conducted as follows: Initially, an appropriate quantity of either WS 2 /WO 3 or pure WO 3 powder was mixed with ethanol to produce a homogeneous slurry.This slurry was then coated onto the external surface of an Al 2 O 3 ceramic tube equipped with two Au electrodes and four Pt wires at both ends, forming a thick sensing film.A Ni-Cr alloy wire, serving as a heater to control the working temperature, was passed through the ceramic tube and welded onto a hexagonal base along with the Pt wires, thus forming the sensor (Figure 1b).Finally, the gas sensor was placed on a TS60 desktop (Winsen Electronics Co., Ltd., Zhengzhou, China) and subjected to aging at 400 • C for 48 h to ensure stability.Gas-sensitive measurements of the prepared sensors were performed under static conditions using a commercial WS-30A system (Winsen Electronics Co., Ltd., Zhengzhou, China).The schematic structure of the gas sensor and the test circuit diagram are depicted in Figure 1b.In this study, a constant loop voltage (V c ) of 5 V was maintained throughout the testing process.The operational principle of the test circuit is governed by the following parameters: R L represents the load resistance, V h denotes the heating voltage, and V out signifies the output load voltage.The ratio of R a (resistance in the air) to R g (resistance in the target gas) is defined as the response.

Structural and Morphological Characteristics
Figure 2 presents the XRD spectra of the samples.It is evident that all XRD patterns of the WO 3 sample exhibit sharp diffraction peaks without any impurity peaks, indicating that the synthesized WO 3 possesses high purity and good crystallinity.The diffraction peaks at 2θ = 23.12• , 23.59 • , 24.38 • , and 34.16 • correspond to the (002), (020), (200), and (202) crystal planes of the monoclinic structure of WO 3 (JCPDS No. 43-1035) [28].Following the sulfurization of WO 3 , diffraction peaks at 2θ = 14.16 • and 33.29 • , corresponding to the (002) and (101) crystal planes of WS 2 (JCPDS No. 97-005-6014) [29], are observed, confirming the formation of WS 2 /WO 3 composites.The intensity of the WS 2 diffraction peaks increases with the increase in the added amount of C 2 H 5 NS, suggesting that more WS 2 is being deposited on the WO 3 surface.In addition, the color of WO 3 products is gradually changed into dark green with the increase in the added amount of C 2 H 5 NS, which also indicates the increased sulfurization degree of WO 3 .
From the SEM images of WO 3 (Figure 3a), it is evident that pure WO 3 shows welldefined flower-like structures with relatively uniform sizes of approximately 4 to 5 µm.Further observation (Figure 3f) reveals that these flower-like structures are composed of many loosely connected nanosheets with a thickness of approximately 10 to 30 nm, which are nearly perpendicular to a central point.This arrangement creates multiple voids between the nanosheets, leading to an increased active surface area and enhanced sensing capability.After in situ sulfurization, the WS 2 /WO 3 nanohybrids retain the flower-like structures, as shown in Figure 3b-e.Additionally, tiny nanoflakes begin to appear on the surface of WO 3 micro-flowers.With the increase in the added amount of C 2 H 5 NS, the distribution of nanosheet-assembled WO 3 micro-flowers becomes denser due to the intensified sulfurization degree of WO 3 and the WS 2 tiny nanoflakes can be well observed (Figure 3h-j).Figure 3k-o presents the EDS element mapping of 5%-WS 2 /WO 3 , which demonstrates the uniform distribution of O, W, and S elements on the surface.This observation confirms the presence of both WO 3 and WS 2 in the samples.
(101) crystal planes of WS2 (JCPDS No. 97-005-6014) [29], are observed, confirming the formation of WS2/WO3 composites.The intensity of the WS2 diffraction peaks increases with the increase in the added amount of C2H5NS, suggesting that more WS2 is being deposited on the WO3 surface.In addition, the color of WO3 products is gradually changed into dark green with the increase in the added amount of C2H5NS, which also indicates the increased sulfurization degree of WO3.From the SEM images of WO3 (Figure 3a), it is evident that pure WO3 shows well-defined flower-like structures with relatively uniform sizes of approximately 4 to 5 µm.Further observation (Figure 3f) reveals that these flower-like structures are composed of many loosely connected nanosheets with a thickness of approximately 10 to 30 nm, which are nearly perpendicular to a central point.This arrangement creates multiple voids between the nanosheets, leading to an increased active surface area and enhanced sensing capability.After in situ sulfurization, the WS2/WO3 nanohybrids retain the flower-like structures, as shown in Figure 3b-e.Additionally, tiny nanoflakes begin to appear on the surface of WO3 micro-flowers.With the increase in the added amount of C2H5NS, the distribution of nanosheet-assembled WO3 micro-flowers becomes denser due to the intensified sulfurization degree of WO3 and the WS2 tiny nanoflakes can be well observed (Figure 3h-j).Figure 3k-o presents the EDS element mapping of 5%-WS2/WO3, which demonstrates the uniform distribution of O, W, and S elements on the surface.This observation confirms the presence of both WO3 and WS2 in the samples.Detailed microstructural analysis of the 5%-WS2/WO3 nanohybrids was conducted using TEM and HRTEM.As shown in Figure 4a, a flower-like structure composed of numerous loosely connected thin nanosheets is observed, consistent with the FE-SEM images.HRTEM images in Figure 4b-d reveal the finer structure and interfaces within the 5%-WS2/WO3 nanohybrids.The lattice fringes at distances of 0.38 nm and 0.26 nm correspond to the (002) crystal plane of WO3 and the (101) crystal plane of WS2, respectively.These observations underscore the distinct crystallization characteristics of the two materials.Specifically, the HRTEM images provide compelling evidence of the formation of WS2/WO3 heterojunctions, which facilitates interfacial electron transfer and consequently enhances the sensing ability.Detailed microstructural analysis of the 5%-WS 2 /WO 3 nanohybrids was conducted using TEM and HRTEM.As shown in Figure 4a, a flower-like structure composed of numerous loosely connected thin nanosheets is observed, consistent with the FE-SEM images.HRTEM images in Figure 4b-d reveal the finer structure and interfaces within the 5%-WS 2 /WO 3 nanohybrids.The lattice fringes at distances of 0.38 nm and 0.26 nm correspond to the (002) crystal plane of WO 3 and the (101) crystal plane of WS 2 , respectively.These observations underscore the distinct crystallization characteristics of the two materials.Specifically, the HRTEM images provide compelling evidence of the formation of WS 2 /WO 3 heterojunctions, which facilitates interfacial electron transfer and consequently enhances the sensing ability.

Gas Sensing Properties
Many factors influence the performance of gas sensors, with operating temperature being particularly significant due to its impact on the adsorption-desorption reaction kinetics of gas molecules on the semiconductor surface [30].To determine the optimal operating temperature, the responses of WO3, 3%-WS2/WO3, 5%-WS2/WO3, 7%-WS2/WO3, and 9%-WS2/WO3 sensors to 10 ppm TMA were evaluated over a temperature range of 25-400 °C.As illustrated in Figure 5a, the response values of all sensors increased progressively with rising temperature, peaking at 200 °C, after which the response values declined with further temperature increments.Thus, the optimal temperature for all sensors is identified as 200 °C, yielding response of 6.58, 16.40, 19.45, 17.67, and 15.29 for WO3, 3%-WS2/WO3, 5%-WS2/WO3, 7%-WS2/WO3, and 9%-WS2/WO3, respectively.Clearly, the introduction of WS2 enhances the sensing response at lower temperature ranges, thereby improving the sensors' reliability and stability for practical applications.
The sensor responses to various TMA concentrations (0.1-100 ppm) are presented in Figure 5b.As the TMA concentration increases from 1 to 10 ppm, all sensors exhibit a rapid response increase.Beyond this range, the response increment slows progressively with higher TMA concentrations, likely due to adsorption saturation of surface active sites [31].Additionally, the 5%-WS2/WO3 sensor consistently exhibits the highest response at each TMA concentration and is capable of detecting TMA at concentrations as low as 0.1 ppm, highlighting the sensitization effect of WS2/WO3. Figure 5c illustrates the responses of sensors to 10 ppm of various interfering gases (TMA, ethanol, methanol, formaldehyde, acetone, and

Gas Sensing Properties
Many factors influence the performance of gas sensors, with operating temperature being particularly significant due to its impact on the adsorption-desorption reaction kinetics of gas molecules on the semiconductor surface [30].To determine the optimal operating temperature, the responses of WO 3 , 3%-WS 2 /WO 3 , 5%-WS 2 /WO 3 , 7%-WS 2 /WO 3 , and 9%-WS 2 /WO 3 sensors to 10 ppm TMA were evaluated over a temperature range of 25-400 • C. As illustrated in Figure 5a, the response values of all sensors increased progressively with rising temperature, peaking at 200 • C, after which the response values declined with further temperature increments.Thus, the optimal temperature for all sensors is identified as 200 • C, yielding response values of 6.58, 16.40, 19.45, 17.67, and 15.29 for WO 3 , 3%-WS 2 /WO 3 , 5%-WS 2 /WO 3 , 7%-WS 2 /WO 3 , and 9%-WS 2 /WO 3 , respectively.Clearly, the introduction of WS 2 enhances the sensing response at lower temperature ranges, thereby improving the sensors' reliability and stability for practical applications.
The sensor responses to various TMA concentrations (0.1-100 ppm) are presented in Figure 5b.As the TMA concentration increases from 1 to 10 ppm, all sensors exhibit a rapid response increase.Beyond this range, the response increment slows progressively with higher TMA concentrations, likely due to adsorption saturation of surface active sites [31].Additionally, the 5%-WS 2 /WO 3 sensor consistently exhibits the highest response at each TMA concentration and is capable of detecting TMA at concentrations as low as 0.1 ppm, highlighting the sensitization effect of WS 2 /WO 3 .Figure 5c illustrates the responses of sensors to 10 ppm of various interfering gases (TMA, ethanol, methanol, formaldehyde, acetone, and ammonia) at the optimal temperature of 200 • C. It is evident that the response of all sensors to TMA is significantly higher than to other gases, indicating good selectivity for TMA.Notably, the 5%-WS 2 /WO 3 sensor shows the highest response to TMA, suggesting its potential application in detecting TMA in complex gas environments.
ing gas (TMA).Conversely, the WS₂ sensor demonstrates a reversed trend in resistance change upon exposure to TMA, indicative of a typical p-type semiconductor characteristic, where the resistance increases in the presence of a reducing gas (TMA).After the removal of TMA gas, all sensors revert to their baseline levels, underscoring their effective response and recovery capacities.The time taken to achieve 90% of the total resistance change during adsorption and desorption processes is termed the response and recovery times, respectively.As shown in Figure 5e, the WS2/WO3 sensor exhibits shorter response and recovery times compared to the WO3 sensor, implying that partial sulfurization-functionalized WO3 enhances the response-recovery characteristics.These findings indicate that the 5%-WS2/WO3 sensor possesses excellent TMA sensing capabilities, which was further investigated at its optimum operating temperature of 200 °C.The transient response and recovery curves of the 5%-WS2/WO3 sensor to different TMA concentrations at 200 °C (0.1-100 ppm) are shown in Figure 6a.When the TMA gas is injected, the resistance decreases rapidly and stabilizes after a short period.Upon removal of TMA from the chamber, the resistance quickly returns to its initial value.This stable and repeatable sensing response behavior, even at lower concentrations (0.1 ppm), suggests its Figure 5d illustrates the response-recovery curves of the WO 3 , WS 2 , and WS 2 /WO 3 sensors.Upon exposure to TMA, the WO 3 and WS 2 /WO 3 sensors exhibit typical n-type semiconductor characteristics, evidenced by a decrease in resistance when exposed to the reducing gas (TMA).Conversely, the WS 2 sensor demonstrates a reversed trend in resistance change upon exposure to TMA, indicative of a typical p-type semiconductor characteristic, where the resistance increases in the presence of a reducing gas (TMA).After the removal of TMA gas, all sensors revert to their baseline levels, underscoring their effective response and recovery capacities.The time taken to achieve 90% of the total resistance change during adsorption and desorption processes is termed the response and recovery times, respectively.As shown in Figure 5e, the WS 2 /WO 3 sensor exhibits shorter response and recovery times compared to the WO 3 sensor, implying that partial sulfurizationfunctionalized WO 3 enhances the response-recovery characteristics.These findings indicate that the 5%-WS 2 /WO 3 sensor possesses excellent TMA sensing capabilities, which was further investigated at its optimum operating temperature of 200 • C.
The transient response and recovery curves of the 5%-WS 2 /WO 3 sensor to different TMA concentrations at 200 • C (0.1-100 ppm) are shown in Figure 6a.When the TMA gas is injected, the resistance decreases rapidly and stabilizes after a short period.Upon removal of TMA from the chamber, the resistance quickly returns to its initial value.This stable and repeatable sensing response behavior, even at lower concentrations (0.1 ppm), suggests its suitability for detecting TMA gas across a wide concentration range.Additionally, the response versus the TMA concentration (0.1-1 ppm) shows a good linear relationship, with a fitting slope of 3.10 ppm −1 (Figure 6b).The theoretical detection limit is calculated to be approximately 15.96 ppb based on the signal-to-noise ratio, indicating its potential application in monitoring ppb-level TMA in the environment.
Nanomaterials 2024, 14, x FOR PEER REVIEW 8 of 13 suitability for detecting TMA gas across a wide concentration range.Additionally, the response versus the TMA concentration (0.1-1 ppm) shows a good linear relationship, with a fitting slope of 3.10 ppm −1 (Figure 6b).The theoretical detection limit is calculated to be approximately 15.96 ppb based on the signal-to-noise ratio, indicating its potential application in monitoring ppb-level TMA in the environment.The repeatability of the 5%-WS2/WO3 sensor was investigated by exposing it to 10 ppm TMA in five cycles of response-recovery at 200 °C.As shown in Figure 6c, each cycle shows similar response and recovery trends, and the initial resistance value shows only slight fluctuations in each trial, implying good repeatability.Moreover, the TMA sensing response of the 5%-WS2/WO3 sensor was measured for 40 days to assess long-term stability.As shown in Figure 6d, although the response values change during the testing process, they degrade only slightly, indicating good stability of the sensor, which is favorable for practical applications.The effect of humidity on gas sensing response was investigated, as depicted in Fig- ure 6e.The response of the 5%-WS2/WO3 sensor to 10 ppm TMA exhibits a slight decrease within the relative humidity (RH) range of 20% to 40%, while it exhibits an obvious decrease at RH levels exceeding 50%.This reduction can be attributed to the competition between surface oxygen and the OH group of H2O for adsorption and reaction sites with TMA molecules [32].Notably, even at a high RH level of 80%, the sensor maintains a response of 8.73 for 10 ppm TMA, implying impressive sensing capability under high-humidity conditions.The operating temperature, response time, and minimum detection limit of various trimethylamine sensors are summarized in Table 1.It can be seen that the 5%-WS2/WO3 sensor has ideal trimethylamine detection sensing characteristics.The repeatability of the 5%-WS 2 /WO 3 sensor was investigated by exposing it to 10 ppm TMA in five cycles of response-recovery at 200 • C. As shown in Figure 6c, each cycle shows similar response and recovery trends, and the initial resistance value shows only slight fluctuations in each trial, implying good repeatability.Moreover, the TMA sensing response of the 5%-WS 2 /WO 3 sensor was measured for 40 days to assess longterm stability.As shown in Figure 6d, although the response values change during the testing process, they degrade only slightly, indicating good stability of the sensor, which is favorable for practical applications.The effect of humidity on gas sensing response was investigated, as depicted in Figure 6e.The response of the 5%-WS 2 /WO 3 sensor to 10 ppm TMA exhibits a slight decrease within the relative humidity (RH) range of 20% to 40%, while it exhibits an obvious decrease at RH levels exceeding 50%.This reduction can be attributed to the competition between surface oxygen and the OH group of H 2 O for adsorption and reaction sites with TMA molecules [32].Notably, even at a high RH level of 80%, the sensor maintains a response of 8.73 for 10 ppm TMA, implying impressive sensing capability under high-humidity conditions.The operating temperature, response time, and minimum detection limit of various trimethylamine sensors are summarized in Table 1.It can be seen that the 5%-WS 2 /WO 3 sensor has ideal trimethylamine detection sensing characteristics.

Gas Sensing Mechanism
In general, metal oxide sensors operate based on the principle that their electrical conductivity changes in the presence of a target gas.In this study, WO 3 and WS 2 /WO 3 sensors exhibit typical n-type sensing characteristics, where electrons are the primary charge carriers in the sensing process.In the air atmosphere, oxygen molecules adsorb onto the surface of the material, capture electrons from the conduction band, and convert into negative oxygen ions (O 2 − , O − , or O 2− ).Consequently, the number of carriers in the material decreases, forming an electron depletion layer on the surface, which hinders electron transfer and increases the sensor's resistance (Figure 7a).When the sensor is exposed to TMA, a reaction occurs between the TMA molecules and the adsorbed oxygen molecules.During this process, the electrons captured by the oxygen molecules are released back to the conduction band, thinning the electron depletion layer and thereby reducing the sensor's resistance in the TMA atmosphere (Figure 7b).The reaction equation is as follows [43,44]: O 2 (gas) → O 2 (ads) O 2 (ads) + e − → 2O − (ads)(100 °C ≤ T ≤ 300 °C) As shown in Figure 7c-e, due to the higher Fermi level of the WO 3 semiconductor, electrons flow from WO 3 to p-type WS 2 , while holes flow from WS 2 to WO 3 until the Fermi level achieves equilibrium [45].The WO 3 surface is an electron depletion region, and the WS 2 surface is a negative charge region.Due to the built-in field, the p-n heterojunction leads to the barrier of electron conduction [46].Upon exposure to TMA, the heterojunctions between WO 3 and WS 2 create additional resistance modulation by altering the potential barriers and the two electron depletion layers.Consequently, the WS 2 /WO 3 heterojunction configuration significantly enhances the sensor's response ability.Secondly, the sensor's performance benefits from the unique flower-like structures, which promote the penetration of oxygen and TMA molecules into the sensing layers, facilitating rapid gas diffusion.Furthermore, the formation of p-n heterojunctions results in the creation of a transition layer, "S-W-O", at the interface [47], which effectively accumulates additional free electrons and facilitates charge transfer.The EIS measurement results (Figure 7f) confirm the enhanced transfer and migration of charge carriers through an internal electric field and interfacial interaction at the heterojunction of WS 2 /WO 3 composites.This increases the carrier density at the interface, generating more adsorbed oxygen and allowing more TMA to participate in the reaction.This results in excellent gas sensing performance.For these comprehensive reasons, the WS 2 /WO 3 sensor exhibits superior sensing performance towards TMA.
This results in excellent gas sensing performance.For these comprehensive reasons, the WS2/WO3 sensor exhibits superior sensing performance towards TMA.

Conclusions
In summary, WS2/WO3 nanohybrids with flower-like hierarchical structures were synthesized via an in situ sulfurization process, employing varying amounts of thioacetamide to control the sulfurization state of WO3.The synthesized WS2/WO3 nanohybrids exhibited significantly enhanced TMA sensing performance compared to pure WO3 micro-flowers.The optimal WS2/WO3 sensor, comprising 5% WS2/WO3 nanohybrids, demonstrated superior sensing response towards TMA, low detection limits, high selectivity, good

Conclusions
In summary, WS 2 /WO 3 nanohybrids with flower-like hierarchical structures were synthesized via an in situ sulfurization process, employing varying amounts of thioacetamide to control the sulfurization state of WO 3 .The synthesized WS 2 /WO 3 nanohybrids exhibited significantly enhanced TMA sensing performance compared to pure WO 3 micro-flowers.The optimal WS 2 /WO 3 sensor, comprising 5% WS 2 /WO 3 nanohybrids, demonstrated superior sensing response towards TMA, low detection limits, high selectivity, good repeatability, and reliable long-term stability.The enhanced sensing performance of the WS 2 /WO 3 nanohybrids is attributed to the formation of p-n heterojunctions at the interface, the unique hierarchical structures, and the catalytic activity of WS 2 .This work offers new insights into the preparation of multifunctional nanomaterials by combining metal oxide micro-flowers with TMD nanoflakes, which hold potential for monitoring TMA in complex atmospheres.

Figure 1 .
Figure 1.Schematic illustrations of (a) the synthesis process of WS 2 /WO 3 nanohybrids, and (b) the gas sensing measurement system.

Figure 5 .
Figure 5.The sensing performance of WO3 and WS2/WO3 sensors: (a) response to 10 ppm TMA at 25-400 °C; (b) response with TMA concentration (0.1-100 ppm) at 200 °C; (c) selectivity to 10 ppm different target gases at 200 °C; (d) dynamic response-recovery curves to 10 ppm TMA (the blue line in each curve represents the response and recovery time interval) and (e) corresponding response and recovery time at 200 °C.

Figure 5 .
Figure 5.The sensing performance of WO 3 and WS 2 /WO 3 sensors: (a) response to 10 ppm TMA at 25-400 • C; (b) response with TMA concentration (0.1-100 ppm) at 200 • C; (c) selectivity to 10 ppm different target gases at 200 • C; (d) dynamic response-recovery curves to 10 ppm TMA (the blue line in each curve represents the response and recovery time interval) and (e) corresponding response and recovery time at 200 • C.

6 .
The sensing performance of the 5%-WS2/WO3 sensor at 200 °C: (a) dynamic response-recovery curves to different TMA concentrations; (b) experimental response signal (blue line) and linear fitted response (pink line) as a function of TMA concentration; (c) eepeatability curves toward 10 ppm TMA; (d) response to TMA over 40 days; and (e) response to 10 ppm TMA under different relative humidity conditions.

Figure 6 .
Figure 6.The sensing performance of the 5%-WS 2 /WO 3 sensor at 200 • C: (a) dynamic responserecovery curves to different TMA concentrations; (b) experimental response signal (blue line) and linear fitted response (pink line) as a function of TMA concentration; (c) eepeatability curves toward 10 ppm TMA; (d) response to TMA over 40 days; and (e) response to 10 ppm TMA under different relative humidity conditions.

Figure 7 .
Figure 7. (a,b) Schematic diagram of the gas sensing mechanism of the WS2/WO3 sensor; (c-e) schematic of energy bands of WS2/WO3 under different conditions; (f) the Nyquist plots of WO3 and 5%-WS2/WO3 were measured in the frequency range of 0.1~10,000 Hz.

Figure 7 .
Figure 7. (a,b) Schematic diagram of the gas sensing mechanism of the WS 2 /WO 3 sensor; (c-e) schematic of energy bands of WS 2 /WO 3 under different conditions; (f) the Nyquist plots of WO 3 and 5%-WS 2 /WO 3 were measured in the frequency range of 0.1~10,000 Hz.

Table 1 .
Comparison of gas sensing properties with other reported sensors to TMA.